As such a kind of conventional threaded fastener, there has been known a full thread bolt as described in a first patent literature, for example. The full thread bolt described in this first patent literature has a screw thread formed on a screw shaft or shank with a length extending up to near a bearing surface of a head, and in addition, also has a “connecting insertion portion” of the same diameter as that of a thread root formed between the head bearing surface and a thread terminal end of an incomplete thread portion.
The “connecting insertion portion” in this first patent literature means an unthreaded neck portion in the vicinity of the head bearing surface, wherein it is constructed such that the root diameter of the incomplete thread portion and the diameter of the neck portion are the same as the thread root diameter so as to keep a screw thread (crest) at the side of an internal screw thread from interfering with the root portion of the incomplete thread portion, so that in cases where the bolt is threadedly engaged with a nut, the head bearing surface of the bolt is placed in intimate contact with a bearing surface of the nut.
However, in the case of the full thread bolt in this first patent literature, for example, as shown in FIG. 3, the incomplete thread portion (denoted by 126) and the neck portion (denoted by 124) of the bolt (denoted by 101) do not interfere with the screw thread of the nut (denoted by 130), but there exists an underneck rounded portion 128 in the base of the neck 124, as a result of which finally, there arises a problem that this underneck rounded portion 128 interferes with an opening edge of an internal screw threaded hole, so that the head bearing surface (denoted by 111) is not completely placed in intimate contact with the bearing surface of the nut 130. The use of a large tightening torque can provide intimate contact therebetween, but there is a fear that the underneck rounded portion 128 or the opening rim or edge of the nut may be damaged.
Accordingly, it can be considered to cut the neck 124 and the underneck rounded portion 128 of the bolt so as to make them slim, but in this case, there is a fear that the strength of the bolt may drop, and moreover, stress may be concentrated on the neck portion 124 to fracture it.